Thermal Design Tool for Outdoor Space Based on a Numerical Simulation System Using 3D-CAD

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1 September 2004 Page 1 of 6 Thermal Design Tool for Outdoor Space Based on a Numerical Simulation System Using 3D-CAD Takashi Asawa 1, Akira Hoyano 1 and Kazuaki Nakaohkubo 1 1 Tokyo Institute of Technology, Yokohama, Japan ABSTRACT: This study developed a thermal design tool for use in planning outdoor spaces by combining outdoor thermal environment simulations with a 3D-CAD system. The input and preprocessing methodology of this tool were developed so that the calculation could be performed using the CAD model that designers draw in the ordinary design process. A method for visually expressing the results of the calculations on 3D-CAD was developed. The results of the application of this simulation tool in an area of detached houses and actual urban block confirms that the simulation tool is able to simulate the effects of different spatial forms and materials, including the effect of shading by tall trees, on the surface temperature distribution and evaluation indexes of outdoor thermal environments. Conference Topic: 2 Design strategies and tools Keywords: thermal environment, outdoor space, thermal design, 3D-CAD, numerical simulation INTRODUCTION The urban heat island phenomenon occurs in urban regions of Japan. In recent years, the deterioration of the urban thermal environment has been recognized as a serious problem during the summer months. Consequently, urban development initiatives that consider the influence of the urban thermal environment have received more attention than they have in the past. Administrators now recognize that heat island mitigation must be an important component of policies directed at urban redevelopment. Such policies would require designers and developers to consider the thermal environment in their architectural designs and plans for outdoor spaces. In addition, a thermally comfortable environment would be pleasant for the inhabitants and commuters in urban areas. However, no environmental design tools are available at present to aid designers and government officials in evaluating a proposed design s potential effects on the thermal environment at the design stage. In this study we developed a thermal design tool for use in planning outdoor spaces by combining outdoor thermal environment simulations with a 3D- CAD system. 2. SEQUENCE OF TOOL DEVELOPMENT The surface temperatures of buildings, trees, and the ground are the primary factors that influence increases in air temperature and the formation of an outdoor thermal radiant field. In a previous study, the authors investigated the relationship between land cover types and their respective surface temperatures using airborne remote sensing. In doing so, we developed a spherical thermography recording system to quantitatively evaluate the relationship between spatial design and thermal radiant field in outdoor spaces [1]. From these studies we illustrated the importance of considering outdoor surface temperature distribution when designing architectural structures and urban blocks, particularly in hot and humid climates such as that experienced during summer in Japan. In order to extend the predictive potential of these findings, we have developed a heat balance simulation model that can predict the surface temperature distribution of an urban area using GIS (Geographic Information System) data [2]. This study improves the previous simulation model, so as to predict the surface temperature distribution of urban blocks taking the actual design of the outdoor space into consideration. This tool was developed to be of use to architects as a thermal design tool for outdoor spaces. 3. DEVELOPMENT OF A SIMULATION TOOL USING 3D-CAD 3.1 Concept of the simulation tool In order to develop a practical simulation tool for widespread application, the following criteria were considered to be essential components of the tool. (1) Users of the system, such as architects and government officials who lack specific knowledge of heat transfer calculations, should be able to use the support tool easily. (2) The calculation should be realized using the CAD models that designers draw in their own design processes. (3) The tool should allow visual communication of the thermal improvements achieved through use of

2 September 2004 Page 2 of 6 the tool to the general public. In addition, users will be able to visually confirm and evaluate the outcome of the simulation. In an effort to satisfy the above conditions, the tool was constructed using 3D-CAD software. 3.2 Outline of the simulation tool Outdoor surface temperatures are dependent upon spatial forms and the materials used to construct outdoor spaces, the ground covering, and the shape and position of trees in the area. In order to evaluate the effects of spatial forms and materials on the outdoor surface temperature, this tool reproduces the detailed shapes and component materials of buildings and trees using a 3D-CAD program. The subject spatial scale of this simulation tool ranges from the size of a single building to the size of an ordinary urban block. The results of surface temperature calculations are then visually projected onto the 3D-CAD model. This allows the user to understand and evaluate the effects of spatial forms and materials on the outdoor surface temperature from almost any viewpoint, including a bird s-eye view and axonometric projections. 3.3 Assessment of the requirements of the simulation tool The following features were examined to develop the simulation tool: (1) The input and pre-processing methodology was developed so that designers could use the tool with no specific knowledge of heat transfer calculations. (2) A method was developed to convert detailed CAD models into the calculation mesh model. (3) Although a smaller mesh size would more accurately depict detailed outdoor spatial forms, the calculation time for the tool with a smaller mesh size is tremendous due to the large number of elements involved in the calculation. An optimal mesh size was thus determined for the tool so as to provide the most detailed spatial form within a suitable calculation time. (4) The simulation algorithm was improved to further reduce the calculation time and memory required for the calculation. This improvement meant that the system could be used on a personal computer. (5) A method for visually expressing the results of the calculations on 3D-CAD was developed. (6) The solar transmission from tree crowns is influenced by tree shape and the course of the solar radiation that permeates through the crown. New tree models were therefore examined and introduced into the simulation system in an effort reproduce their solar transmission characteristics more accurately. 3.4 Input and pre-processing methodology on 3D-CAD The input and pre-processing methodology of this tool were developed in such way that the calculation could be performed using the CAD model that designers draw in the ordinary design process. Fig.1 shows the input and pre- processing method of this simulation tool. Fig.2 is a flow chart depicting the various components and processes involved with this tool. Building and tree shapes are drawn using allpurpose 3D-CAD software. The input operation system is based on a GUI (Graphical User Interface), so that users can input or select component materials and building member using a dialog. The following data are stored in the "Spatial Component Database": (a) Buildings: the building structure, building member, material, and surface colour (b) Trees: the species (c) Ground: land cover and its composition The data inputted in the above process are all design parameters, so that user can make this CAD model without any specialist knowledge. The parameters for the calculation, physical properties of materials and heat transfer calculation models, are then automatically selected and determined from the aforementioned design parameters and other databases. The physical properties of materials are stored in the "Material Database". The objects that require the specific modelling of heat transfer, for example rooftop lawnplanting, are also stored in the Heat Transfer Calculation Model Database. Building Member Roof Veranda Database Spatial component D.B. CAD Model Mesh Model Transforming into Mesh Model for calculation Wall Pane Wooden Deck and so on link Material D.B. Physical properties of material Heat transfer calculation model D.B. Figure 1: The input and pre- processing method of this simulation tool

3 September 2004 Page 3 of 6 The CAD models generated by this process are then transformed into a mesh model that includes the calculation parameters required for heat transfer analysis. 3.5 Heat balance calculations and the evaluation of simulation results Heat balance and one-dimensional heat conduction are then performed for each mesh (Fig.3). The heat balance elements calculated in this process are direct solar radiation, sky solar radiation, reflected solar radiation, atmospheric radiation, long wavelength radiation, convective heat transfer, and heat conduction. Sky solar radiation and atmospheric radiation are calculated from the sky factor of each mesh. Convective heat transfer is calculated under the assumption that there is no distribution of air temperature and wind velocity in the subject outdoor space. The weather data used for this calculation are the vertical quantity of total solar radiation, air temperature, relative humidity, wind velocity, and cloud amount. Indoor air temperature is inputted as a boundary condition of indoor side. The calculation results are visually projected onto the 3D-CAD model. The present tool evaluates the impact of a proposed design on the thermal environment of outdoor space by the indices using surface temperature. The evaluation indices include the surface temperature, the Mean Radiant Temperature (MRT; an important factor for thermal comfort in outdoor space), and the Heat Island Potential (HIP) [2], which reflects the total heat flux from the entire surface of the urban block being analyzed. The prediction and evaluation that include air flow, air temperature and humidity will be examined and incorporated in the next study. INPUT Making of urban block model using 3D-CAD program Taking 3-dimentional spatial coordinates data from 3D-CAD model Spatial Component Database Buildings: building structure, member, material, surface color and so on Trees: species, solar transmittance and so on Ground: land cover, materials and so on Material Database Physical properties of material: thermal conductivity, volumetric specific heat, solar reflectance, longwave emittance SOLVER Transforming CAD model into mesh model for calculation Heat Receiving Flux on each Surface Direct Solar Reflected Solar (Specular, Diffusion) Sky Solar Atmospheric Convective Heat Transfer 1-dimentional heat conduction calculation Calculation of Surface Temperature OUTPUT Displaying of Surface Temperature Distribution onto 3D-CAD Heat balance calculation for each mesh Normal direction of the mesh surface, Solar absorptivity, The time, Longitude, Latitude Direct solar radiation incident on a normal surface Sky solar radiation incident upon a horizontal surface Sky Factor Brunt s formula Cloud Amount Atmospheric radiation (unobstructed sky) Longwave emittance of each mesh surface MRT Distribution at a Height of 1.5m Heat Transfer Calculation Model Database rooftop lawn-planting, ivy covered wall, louver and so on Separation of direct and diffuse components of radiation Weather Condition Database Vertical Quantity of Total Solar Relative Humidity Air temperature Convective heat Wind Velocity transfer coefficient Convective heat transfer Jurges s formula coefficient of indoor, indoor air temperature Long Wavelength Longwave emittance HIP (Heat Island Potential) Figure 2: The Flow Chart of This Simulation Tool Building Sky Solar Atmospheric Heat Conduction Sky Factor of Each Mesh Solar (Direct,Sky,Reflected) Long Wavelength Convective Heat Transfer Reflected Solar Tree Solar Transmission Reflected Solar (Specular, Diffusion) Direct Solar The shape of tree model is made on the 3D-CAD software The tree crown is filled with the meshes, and then the solar transmittance is allotted to each mesh. Figure 3: Mesh Model of Building and Tree, and Heat Balance Calculation at Each Mesh

4 September 2004 Page 4 of 6 HIP is calculated by the following formula: ( Ts Ta ) ds all surfaces HIP = _ A (1) HIP Heat Island Potential [ C] T s Surface temperature of each mesh [ C] T a Air temperature[ C] A Plane square measure of the urban block [m 2 ] ds Square measure of each mesh [m 2 ] 4. SIMULATION TOOL APPLICATIONS 4.1 Application to actual urban blocks This tool is intended for application in architectural design and urban planning at the level of the block. This subsection describes the application of this tool on actual urban blocks in Tokyo. Fig.4 depicts the CAD model of the urban blocks examined and Fig.5 depicts the surface temperature calculation results of the urban block. The mesh size of this calculation is 0.4m. The calculation time for this subject area, which include the high-rise buildings, is approximately 16 hours using a PC (Macintosh power Mac G4 1.25GHz Memory:1Gbyte). The calculation time for the residential area taken up in the next subsection is approximately 2hours. The effect that high-rise buildings have on the surface temperature distribution on the surrounding area, as well as the effect of the different component materials, is clearly illustrated by Fig. 5. Consequently, these results indicate that this tool is capable of predicting the effect that spatial form and the materials of urban blocks have upon the urban thermal environment. 4.2 Evaluation of the effect of planting tall trees using this tool In order to verify the validity of the simulations for purposes of thermal design in outdoor areas, the simulation tool was applied to an area with detached houses surrounded by trees with leafy canopies. The site was chosen so that the effect of tall trees on controlling the thermal environment could be examined. Five two-story houses of a residential area in the Kanto region were selected to evaluate the applicability of the tool to outdoor thermal environments. The detailed weather conditions used in the calculation were taken as a summer day with clear skies. The calculation was performed for three different scenarios and CAD models for each case are depicted in Fig. 6. In the first scenario, there were no trees in the area (CASE1). CASES 2 and 3 differ in the number and positions of trees, and include trees that are taller than the houses. The morphology of the trees was modelled on the Zelkova genus. The mesh size of this calculation is 0.2m. Figures 7 shows a bird s-eye view of surface temperature distribution depicted in the 3D-CAD model, as well as a perspective depiction of surface temperature distributions. (a) Bird s-eye view of surface temperature distribution [ C] Air temp. 25 N Buildings for boundary condition [ C] Subject calculation area 0 Figure 4: CAD model of actual urban blocks for calculation 100 [m] (b) Perspective depiction of surface temperature distribution Figure 5: Simulation results of surface temperature distribution of the urban blocks (Clear sky day in summer, 12:00) 35 Air temp. 25

5 September 2004 Page 5 of 6 <CASE1: There is no trees> <CASE2: The present condition> <CASE3: There are many tall trees> Figure 6: CAD models of the residential area for calculation <CASE1: There is no trees> <CASE2: The present Condition> <CASE3: There are many tall trees> MRT40 C MRT36 C MRT32 C [ C] [ C] [ C] Air temp. Air temp. Air temp. <CASE1: There is no trees> <CASE2: The present condition> <CASE3: There are many tall trees> Figure 7: Simulation results of surface temperature of the residential area (Clear sky day in summer, 12:00) [ C] Air temp C Mean Radiant Temperature CASE2 12:00 Figure 8: MRT distribution at a height of 1.5m Heat Island Potential HIP [ C] Asphalt pavement CASE1 :No tree CASE2 :Present condition lawn CASE3 :Planting tall trees Time Figure 9: Diurnal Change of HIP (Clear sky day in summer)

6 September 2004 Page 6 of 6 In CASE1, which has no trees, the surface temperatures of roofs and the ground were observed to exceed 50 C due to being directly exposed to solar radiation. Conversely, in CASE3, the effect of shade from the tall trees resulted in many surfaces having temperatures almost equivalent to ambient air temperature. Figure 8 depicts the MRT distribution at a height of 1.5 m. MRT under the tall trees was 32 C, which was approximately 10 C lower than that observed in areas exposed to sun. Figure 9 depicts the diurnal change in the HIP. This figure indicates that the HIP drops markedly when tall trees have been planted. The difference in the HIP between CASE1 and CASE3 at noon is approximately 15 C. This difference is almost the same as the difference observed in comparisons between asphalt pavement and lawn. These results reveal that the model is able to simulate the effects of building shapes and the effect trees shade on surface temperature, and the MRT and HIP indexes. Therefore, this simulation system has effectively demonstrated its potential utility as a means of evaluating the impact of a proposed design on the thermal environment of an outdoor space. This study was also supported in part by Grantsin-Aid (2003) from NEDO (New Energy and Industrial Technology Development Organization). REFERENCES [1] Kohichi Asano and Akira Hoyano: Application of a new spherical thermography technique to monitoring outdoor long wave radiation fields, Proceeding of SPIE, the Society of Photo-Optical Instrumentation Engineers, Vol. 3436, pp , [2] Akinaru Iino and Akira Hoyano: Development of a method to predict the heat island potential using remote sensing and GIS data, Energy and Building 23, pp , CONCLUSION This paper details the development of a simulation tool that will enable a user to virtually predict and evaluate the effect their building designs will have on an area's thermal environment using 3D- CAD software. The following features were examined and incorporated into this simulation tool to allow for the consideration of detailed outdoor spatial forms and to put the system to practical use: input and preprocessing methodology that uses 3D-CAD, an optimum mesh size, a calculation algorithm that reduces the calculation load, and a method to visually inspect the results of the calculation. The results of the application of this simulation tool in an area of detached houses confirms that the simulation tool is able to simulate the effects of different spatial forms and materials, including the effect of shading by tall trees, on the surface temperatures and evaluation indexes of outdoor thermal environments. Future research will focus on improving the input database, specifically, to include greening methods. A latent heat model will also be introduced to evaluate the effect of rainfall and evaporative cooling. And this system will be combined with CFD so as to predict the air flow and air temperature distribution in an area. ACKNOWLEDGEMENTS This study was supported in part by Grants-in-Aid for Scientific Research (B), No (2002, 2003) from the JSPS (Japan Society for the Promotion of Science).